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FATTY SUBSTANCES  (GENERAL METHODS)            377

the ester number is given by the difference between the acid and saponifi-
cation numbers.

The ester number has importance especially for the analysis of beeswax.

10. Volatile Acid Number (Reicbert-Meissl Number)

By this is meant the number of c.c. of decinormal alkali necessary to
neutralise the volatile acids, soluble in water, obtained from 5 grams of a fatty
substance under definite conditions.

Reichert's original method has been modified in various ways, the modi-
fication proposed by Leffmann and Beam and by Polenske being now
preferred.

In a flat-bottomed, 300 c.c.
flask of resistant glass (.4),
exactly 5 grams of the fatty
substance^ (filtered oil or mol-
ten fat) are heated over a
small flame with 20 grams of
pure glycerine and 2 c.c. of
caustic soda solution (100
grams of pure sodium hydro-
xide in 100 grams of water)
until the liquid froths and
becomes clear and homoge-
neous (5-8 minutes). The
liquid soap thus obtained is
allowed to cool to 80-90 and
is then shaken with 90 c.c. of
distilled water at the same
temperature, and heated if
necessary, until solution is
complete. 50 c.c. of dilute
sulphuric acid (25 c.c. of the
pure cone, acid to the litre)
and 0-6-0-7 gram of roughly
powdered pumice are then
added and the liquid distilled                             FlG- 53

into a no c.c. flask.

For this distillation the condenser and accessory apparatus have the form
and dimensions (m.m.) shown in Fig. 53. The distilling flask is supported
on an asbestos card on a ring 6-5 cm. in diameter, and the flame should be
such that the no c.c. of distillate are collected in about 19-21 minutes.

When the no c.c. of distiUate are collected, the flame is extinguished
and the collecting flask replaced by another vessel and left in water at 15
without shaking for 10 minutes. It is then stoppered, inverted four or five
times to mix the liquid but not to shake it too much, and filtered through
a dry filter 8 cm. in diameter, 100 c.c. of the filtrate being titrated with
decinormal potassium hydroxide solution in presence of 3-4 drops of i%

m

d oil, and a few other oils, have values below 190.